The invention relates generally to alarm systems for automobiles and more specifically to a method and apparatus for detecting the breakage of the windows of an automobile.
In an automotive vehicle it is desirable to activate an alarm in response to theft attempts as determined by certain sensed inputs. For example, the unauthorized opening of a cabin door or trunk lid triggers an alarm. Another sensed input desirable for triggering an alarm is glass breakage. Glass breakage has been sensed using a frangible screened trace (or patch) of electrically conductive material deposited on the glass. Security systems in buildings such as banks use such screened traces. However, in a building, the exterior wall at the periphery of the glass cannot be easily penetrated in order to access and tamper with the trace. In a motor vehicle, only sheet metal is present at the periphery of the glass. If the patch on glass systems typically employed on a building were employed on an automobile, the wiring of such systems might be reachable from the outside of the automobile by piercing the external sheet metal. Since the wiring can easily be reached by would-be intruders, previously known systems can easily be defeated by selectably shorting wires together.